1. Technical Field
The presently disclosed subject matter relates to a semiconductor device having a plurality of semiconductor elements mounted thereon, to a semiconductor device module having such a semiconductor device mounted thereon, and to a method for manufacturing such a semiconductor device module.
2. Description of the Related Art
Semiconductor devices (such as semiconductor light emitting devices) including a plurality of semiconductor elements (such as semiconductor light emitting elements) that serve as light sources are used in various fields and applications such as light sources for backlights of LCD display devices, light sources for vehicle interior illumination devices, light sources for outdoor illumination devices, and light sources for flash lamps. Accordingly, the performance specifications of such semiconductor devices can vary for different applications.
If such light emitting devices have high brightness, semiconductor devices can be configured such that the luminous or radiant flux of light emitted from each of the semiconductor light emitting elements is increased, or such that the number of the semiconductor light emitting elements constituting the light emitting device is increased to increase the luminous flux of light emitted from the light emitting device.
The former case can be achieved by increasing the size of each semiconductor light emitting element to increase the area of the light emitting surface (or to increase the flux of the semiconductor light emitting element). However, the increase in the size of the semiconductor light emitting element can cause the following problems:
(1) The probability that crystal defects such as dislocations are present can be high, and therefore the characteristics in a small-current region and the reverse characteristics are likely to deteriorate;
(2) The number of semiconductor light emitting elements that can be obtained from one wafer can be low, and the manufacturing yield, or the number of usable elements, can be low, so that the manufacturing cost can increase; and
(3) The linearity can deteriorate due to the potential distribution in the junction plane caused by the layer resistance.
To solve these problems, a plurality of semiconductor light emitting elements of ordinary size can be used to increase the luminous intensity of the light emitting device, although the manufacturing cost including the material cost and assembling cost can increase.
In such a case, the plurality of semiconductor light emitting elements are electrically connected, for example, in series or parallel. When the semiconductor light emitting elements are connected in series, the same current can flow through each of the semiconductor light emitting elements irrespective of variations in the current-voltage characteristic of the elements, so that the variations in the luminous or radiant flux of the elements can be suppressed. However, problems of series connection can be when a high voltage power source is used, a high driving voltage occurs and that if even one of the semiconductor light emitting elements is broken, then all the elements do not illuminate.
When the semiconductor light emitting elements are connected in parallel, the driving voltage can be low, and therefore the elements can be driven by a low-voltage power source. In addition, even when one of the semiconductor light emitting elements is broken and does not illuminate, the rest of the elements can be illuminated. However, a problem of parallel connection can be that the values of the currents flowing through the semiconductor light emitting elements are different due to variations in their current-voltage characteristics. This can cause variations in the luminous or radiant flux of the semiconductor light emitting elements and variations in luminous or radiant flux reduction ratio.
As described above, each of the series and parallel connections of semiconductor light emitting elements has advantages and disadvantages. Generally, in many cases, semiconductor light emitting devices are used together with other electronic components such as transistors and ICs and driven by general-purpose simple power sources such as dry batteries. Therefore, a parallel connection, in which a low-voltage power source can be used, is often employed.
When semiconductor light emitting elements are connected in parallel, the problem associated with the parallel connection must or should be solved. To solve the problem, the following methods have been proposed.
In one method, semiconductor light emitting elements to be connected in parallel are selected in advance such that their forward voltages match each other (see, for example, Japanese Patent Application Laid-Open No. 2006-222412 corresponding to US 2006/0171135A1). In this manner, the variations in the forward voltages of the semiconductor light emitting elements are reduced to render the forward currents flowing through the elements uniform, so that the variations in luminous flux and variations in luminous flux reduction ratio are reduced.
In another method, semiconductor light emitting elements are die-bonded with die-bonding paste having a resistive component (see, for example, Japanese Patent Application Laid-Open No. 2006-339541). The die-bonding paste provides a series resistance according to the forward voltage of each semiconductor light emitting element, so that the current imbalance between the elements is reduced.
In another method, semiconductor light emitting elements are wire-bonded with bonding wires having a certain resistance (see, for example, Japanese Patent Application Laid-Open No. 2006-339540). In this manner, the gradient of voltage-current characteristics of the semiconductor light emitting elements is reduced to decrease the current imbalance between the elements.